1. Field of the Invention
This invention relates to polymeric compositions and the use of these compositions in electrophotographic elements and processes. More specifically, this invention involves random copolymers suitable for use in electrophotographic imaging members and processes. The spatial constraint, relative conformation, relative ionization, potential and relative electron affinity of the pendant groups of the two principal components of these compositions favors charge transfer interaction between them resulting in highly colored polymeric materials.
2. Description of the Prior Art
The formation and development of images on the imaging surfaces of photoconductive materials by electrostatic means is well known. The best known of the commercial processes, more commonly known as xerography, involves forming a latent electrostatic image on an imaging surface of an imaging member by first uniformly, electrostatically charging the surface of the imaging layer in the dark and then exposing this electrostatically charged surface to a light and shadow image. The light-struck areas of the imaging layer are thus rendered relatively conductive and the electrostatic charge selectively dissipated in these irradiated areas. After the photoconductor is exposed, the latent electrostatic image on this image-bearing surface is rendered visible by development with a finely divided colored marking material known in the art as "toner." This toner will be principally attracted to those areas on the image-bearing surface having a polarity of charge opposite to the charge on the toner particles, thus forming a visible powder image.
The developed image can then be read or permanently affixed to the photoconductor where the imaging layer is not to be reused. This latter practice is usually followed with respect to the binder-type photoconductive films (e.g. ZnO dispersed in a resinous binder) where the photoconductive imaging layer is also an integral part of the finished copy.
In so-called "plain paper" copying systems, the latent image can be developed on the imaging surface of a reusable photoconductor or transferred to another surface, such as a sheet of paper, and thereafter developed. When the latent image is developed on the imaging surface of a reusable photoconductor, it is subsequently transferred to another substrate and then permanently affixed thereto. Any one of a variety of well known techniques can be used to permanently affix the toner image to the copy sheet, including overcoating with transparent films, and solvent or thermal fusion of the toner particles to the supportive substrate.
In the above "plain paper" copying system, the materials used in the photoconductive layer should preferably be capable of rapid switching from insulating to conductive to insulating state in order to permit cyclic use of the imaging surface. The failure of a material to return to its relatively insulating state prior to the succeeding charging sequence will result in a decrease in the maximum charge acceptance of the photoconductor. This phenomenon, commonly referred to in the art as "fatique," has in the past been avoided by the selection of photoconductive materials possessing rapid switching capacity. Typical of the materials suitable for use in such a rapidly cycling system include anthracene, sulfur, selenium and mixtures thereof (U.S. Pat. No. 2,297,691); selenium being preferred because of its superior photosensitivity.
In addition to anthracene, other organic photoconductive materials, most notably, poly(N-vinylcarbazole), have been the focus of increasing interest in electrophotography. Most organic photoconductive materials, however, including poly(N-vinylcarbazole), lack the inherent photosensitivity to be competitive with selenium. This need for the enhancement of the photoresponse characteristics of organic photoconductors thus led to the formulation of these organic materials with other compounds, commonly referred to as "activators." Poly(vinylcarbazoles), for example, when sensitized with 2,4,7-trinitro-9-fluorenone exhibit good photoresponse and discharge characteristics and, (depending upon the polarity of the surface charge), low dark decay; U.S. Pat. No. 3,484,237. Other organic resins, traditionally considered nonphotoconductive can also be sensitized with certain activators, such as Lewis Acids, thus forming charge transfer complexes which are photoresponsive in the visible band of the electromagnetic spectrum, U.S. Pat. Nos. 3,408,181, 3,408,182 3,408,183, 3,408,184, 3,408,185, 3,408,186, 3,408,187, 3,408,188, 3,408,189 and 3,408,190. With respect to both the photoconductive and nonphotoconductive resins, the degree of sensitization is generally concentration dependent; the higher the loadings of activators, the greater the photoresponse.
The concentration of activator capable of formulation with the above materials, however, is finite; generally being limited to less than 10 weight percent of the composition. Ordinarily, the addition of high loadings of activator to many of the above materials will lead to impairment of mechanical and/or the photoconductive properties of the sensitized composition. In most instances, the excessive addition of activators to both the photoconductive and nonphotoconductive materials of the types disclosed in the above patents will result in crystallization of these activators, thus impairing the mechanical strength and other physical properties of the resultant photoconductive composition. Still yet other sensitizers, when present in relatively low concentration can result in oversensitization of composition in that the photocurrents generated upon exposure will persist long after illumination ceases, BUL. CHEM. SOC. of JAP. 39, 1660 (1966). This phenomenon prevents the further use of such materials for preparation of successive electrostatic reproductions until such persistent conductivity is dissipated in the previously illuminated areas of the photoconductor. The dissipation of persistent photocurrents generally takes an extended period of time and/or thermal erasure, thus making these oversensitized compositions generally unsatisfactory for rapid cycling electrostatographic imaging systems.
As an alternative to the more traditional type of sensitization discussed above, Inami and Morimoto have proposed preparation of "intramolecular" charge transfer complexes (more properly characterized as "intrachain" charge transfer complexes) wherein the electron donor and electron acceptor substituents are located along a common vinyl backbone, U.S. Pat. No. 3,418,116. The materials of principal interest disclosed in the above patent are the nitrated vinyl polymers of polyacenaphthylene, poly-9-vinylcarbazole and poly-1-vinylnaphthalene. Intrachain charge transfer complexes have also been disclosed by Podhajny (U.S. Pat. No. 3,697,264) and Limburg (U.S. Pat. No. 3,877,936). All of the intrachain charge transfer complexes disclosed to date comprise relatively strong electron donor structural units and relatively weak electron acceptor structural units. Attempts at preparation of monomers having strong electron acceptor groups (groups having an electron affinity in excess of about 0.7 electron volts) have up to now been generally unsuccessful. Even in the limited instances where it has been possible to prepare such monomers, polymerization of these monomers by free radical initiation has been virtually impossible, since the electron acceptor moiety quenches the free radical. Introduction of strong electron acceptor substituents onto preformed polymer backbones has also encountered considerable difficulty. For example, attempts at nitration of poly(vinylfluorenone) results in degradation in the polymer chain and reduction in its solubility in common solvents (presumably due to cross-linking).
Accordingly, it is the object of this invention to remove the above as well as related deficiencies in the prior art.
More specifically, it is the object of this invention to provide a method for enhancement of electron donor and electron acceptor interaction so as to increase the probability of charge transfer complex formation.
It is the primary object of this invention to provide a method and means for achieving free radical initiated copolymerization of an addition monomer having pendant therefrom strong electron donor groups with an addition monomer having pendant therefrom strong electron acceptor groups.
It is another object of this invention to provide a polymeric composition capable of formation of an intrachain charge transfer complex.
It is yet another object of this invention to provide a copolymer composition having a random distribution of electron acceptor and electron donor structural units.
Additional objects of this invention include dilution of the above random copolymer so as to minimize charge transfer interaction and yet maintain the ambipolar transport properties of the composition.